1. Field of the Invention
The present invention relates to a sequence control apparatus which can easily investigate causes of abnormality in a sequence control system.
2. Description of the Prior Art
It has been proposed that in the case where abnormality occurs in the sequence control during the operation, the state transitions of input and output are held and the causes of the occurrence of the abnormality are investigated.
According to the method proposed conventionally, the states of the input/output signals of the sequencer are time-sequentially stored when abnormality occurs, and the memory content is printed on a paper or displayed on a screen or the like, and is compared with a check list which has been preliminarily stored and printed on the paper or displayed on the screen in the normal operating mode.
This method is effective for investigation of the causes of abnormalities in the case where it is used for a system to be controlled in which the input/output signals change at accurate time intervals in accordance with an accurate time sequence.
However, in many systems to be controlled, the input/output signals do not change at accurate time intervals.
In such a case, it is difficult to investigate the causes of the abnormality by time-sequentially following the states of the input/output signals. This is because the conditional judgment type sequencer takes in the input/output signals synchronously with the timing signal irrespective of the transition of the input/output signals, so that even if the input/output signals change, this change is stored when the scanning operation is performed instead of being stored at the time of the occurrence of the change.
The failure diagnosis function has been set forth in the Official Gazette of Japanese patent Examined Publication No. 20042/1983.